(1) Technical Field
This invention generally relates to electronic power supplies, and more specifically to capacitive energy transfer DC-to-DC converters (DC/DC converters), such as charge pumps.
(2) Background
DC/DC converter power supply circuits provide a DC output voltage based upon a DC source voltage, where the output voltage is typically different than the input voltage. As the term is used herein, DC/DC converters do not encompass voltage reduction regulator circuits that use a linear pass device, but rather involve energy transfer from input to output through an energy storage device, such as a capacitor or an inductor.
A type of DC/DC converter is a “charge pump”, which obtains energy for the output voltage primarily by means of capacitive transfer from the source to the output. An inductor is not generally the primary energy transfer device in a charge pump, though of course hybrid devices are possible that employ inductive energy transfer in addition to capacitive energy transfer. A charge pump may derive an output voltage that is higher than a source voltage, or that is inverted from a source voltage, or that is referenced to a different voltage than the source voltage, and may do all of these things concurrently. Energy transfer capacitors used in charge pumps are typically known as “fly capacitors” or “fly caps”.
Charge pumps may be implemented for a wide variety of purposes. They are well suited for integrated circuit fabrication because the devices and elements required are compatible with most integrated circuit fabrication techniques. For example, a charge pump may be employed to generate a negative gate bias supply for an integrated circuit that switches an antenna between send and receive circuitry of a transceiver, as shown in FIG. 1. Many wireless transceivers, such as cellular telephones, employ a single antenna for both receiving and transmitting. While such systems are receiving, an antenna 102 must be coupled to receive circuitry 103 that may include, for example, a filter 104 and a low noise amplifier 106, to provide the received signal for further processing. However, while such systems are transmitting, the antenna 102 must be disconnected from the sensitive receive circuitry 103 and coupled instead to relatively high power transmit circuitry 107. The transmit circuitry 107 may further include, for example, a power amplifier 108 and a transmit filter 110 to process a transmit signal. Note that the circuit shown in FIG. 1 is schematically simple for ease of understanding; in an actual implementation, there are often multiple transmit and receive circuits, and transmission and reception may be occurring on the same path at the same time.
An RF switch 112 may be used to perform such antenna switching functions, as well as RF switching functions in general. Ideally, such switches may be integrated together with the receive and/or transmit circuitry, and in any event are desirably very small, due to integrated circuit die cost and space limitations in portable transceivers such as mobile telephones and handy talkies. In order to achieve good performance from switching devices, such as FETs, used to implement such RF switches, many designs need a special bias supply that extends negatively below the supply rails of the transmit and receive circuitry, such as a −3V supply. In view of the space and cost constraints of transceiver units such as mobile telephones, a charge pump is particularly suitable for generating such a bias supply, because it can be readily integrated into a very small circuit.
The RF switch 112 conveys relatively high power signals to the antenna 102 during transmission. However, during receive, the signal passed by the RF switch 112 may be measured in tens of nanovolts. Sharp noise transitions may have an extremely broad frequency content, and thus even signals at amplitudes on the order of millivolts may interfere unacceptably with reception if the signals have extremely fast edges. While the filter 104 can remove some noise, it is important that the RF switch 112 not introduce noise, particularly noise having components near the center frequency of the received signal. Thus, the receive/transmit switch of FIG. 1 illustrates one of many circumstances in which a charge pump may be desired for a circuit that nonetheless requires extremely low noise.
Unfortunately, noise generation is one of the most common drawbacks of charge pumps. Current spikes are typically coupled into both input and output supplies, together with voltage ripples and spikes. When a charge pump is integrated together with other devices, such electronic noise may be coupled throughout the circuitry of the integrated device by a variety of mechanisms that are difficult to control.
Charge pump power supplies can also be weak (i.e., not able to drive large load currents), although a designer may trade drive strength for noise. In some applications (generally FET based designs), loading events primarily occur during state or mode changes for the part. During this time, the noise generated by the charge pump circuitry is not a critical factor. As such, a designer may desire some way to switch between a mode that is strong, and one that is quiet. Charge pumps typically require the use of some form of clock, whether externally provided or internally generated. A higher clock rate will make for a stronger charge pump, but this may also introduce more noise. Also, higher frequency noise terms may tend to couple more easily into undesired places or bands. In particular, the frequency of the charge pump clock can show up as a distinct “spur” signals (spurs) both at multiples of itself in baseband and/or at multiples of itself offset from whatever RF frequency is being utilized by the system. In the case of a switch with both transmit and receive signals present in different bands, it is possible for clock spurs offset from the transmit band to show up in the receive band.
It is known to use two clock frequencies, high and low, in a charge pump circuit to switch between a strong, noisy mode and a weak, less noisy mode. However, in such designs, the clock frequency is only high for a fixed period of time starting at the beginning of a state change. Due to process variation and unknown loading conditions, this two-state clock may either turn off too soon, resulting in poor settling time, or too late, resulting in noise being present during a desired “quiet” period.
Thus, a need exists for charge pumps that avoid generating excessive noise, so as to reduce charge pump noise injection into source supplies, output supplies, and related circuits.
The method and apparatus presented below address this need for a low-noise charge pump. Various aspects of the method and apparatus described below will be seen to provide further advantages, as well, for the design and construction of charge pumps that are relatively free of noise spurs.